It is known that all physical bodies at temperatures above absolute zero emit electromagnetic radiation whose spectral radiance L at a given wavelength [.lambda.] is described by the Planck formula: EQU L=E2hc.sup.2 .lambda..sup.-5 [exp {hc/.lambda.kT}-1].sup.-1
where E is the spectral emittance of the radiating object at this given wavelength; T is the absolute temperature of the object; c is the velocity of light in vacuum; h is the Planck constant; and k is the Boltzmann constant.
It is also known that the spectral emittance E at a given wavelength [.lambda.] of opaque bodies can be determined from a measurement of the directional spectral hemispherical reflectance R at the same given wavelength in accordance with the Kirchhoff principle: EQU E=!-R,
The directional spectral hemispherical reflectance is defined as the relative radiance power reflected back into hemispherical space when the surface is irradiated by a collimated beam of radiation incident from a particular direction [.phi.,.theta.], where .phi. is the azimuth and .theta. is the elevation angle relative to the surface normal. The emittance value E in the Planck formula is that observed in the same direction.
When the values for directional spectral hemispherical reflectance and spectral radiance can be measured directly, the surface temperature of any opaque object can be determined accurately by using the Planck formula and the Kirchhoff principle without touching or interfering in any way with the object of interest. Such measurements have been made by placing a specimen in a specially designed integrating sphere reflectometer and employing laser beam heating and monochromatic irradiation of the specimen with modulated laser light. A detailed description of this procedure is contained in a paper entitled MEASUREMENTS OF SPECTRAL EMISSIVITY OF UO.sub.2 ABOVE THE MELTING POINT, by M. Bober and H. U. Karow [Proceedings of the Symposium on Thermophysical Properties, 7th series, U.S. Bureau of Standards; published by Am. Soc. of Mech. Engineers, NYC 1977, pages 344-350].
However, the apparatus described in this paper can only be used with a specimen that can be placed within the integrating sphere. There is a need for an apparatus which can be used to make the same measurements on large objects which cannot be disposed in an integrating sphere.
The present invention is directed toward new portable relatively inexpensive apparatus which can be used not only to obtain a measurement of the directional spectral hemispherical reflectance of an opaque object, but also its temperature, regardless of the surface texture of the object and without contacting or interfering in any way with the object itself.